TSUGI is a very interesting approach to building a NGDLE (Next gen digital learning environment). It is another in a long line of fine projects by Chuck Severance, who is one of the leaders of Sakai (last gen digital learning environment), and who has created some of the most successful (in every sense) MOOCs for programming ever made. This consequently has a very good pedigree and a very sound design, with support not only from the indefatigable Chuck himself but also his very large network. 

Chuck describes TSUGI as an LMS without the interface, Blackboard Building Blocks without Blackboard, Sakai Tools with no Sakai (Moodle Modules with no Moodle) conceptually similar to an app store from an end-user perspective. This is the same kind of design pattern that is used by Elgg, the framework behind the Landing, taking it an important step further by enabling end users to create the environments they need for learning, not just system administrators.

It's essentially a container for apps that lets teachers assemble those needed, and only those needed, from a potentially wide range of tools, offering the underlying services needed to make sure that they seamlessly interoperate. So, all those one-size-fits-all options that are available in last gen LMSs become separate, replaceable apps. Don't like Moodle's discussion forum? Replace it with another. Need to add a webmeeting? Just select it and it is there. The same goes for content, quizzes, assessments, gradebooks, etc, etc.  It also integrates neatly with existing tools (Sakai and Canvas are supported, Moodle may take a while but will get there in the end, others are on the way) so it provides a smooth incremental upgrade path from existing infrastructures without loss of existing investments.

TSUGI is open source and aggressively leverages open standards, including (notably) LTI 1 and 2, Caliper, xAPI, Common Cartridge, and many more. It allows individuals or institutions to plug in any compliant tool, knitting them together in infinitely many ways. It is also relatively easy for developers to create new tools for end users to use, packaged in a way that should not bring down the whole system when they fail.  What Chuck has done is essentially to take the hard work out of implementing complex e-learning standards and packaged it all together in a form that makes modular development sustainable and safe. It's an excellent design pattern that I and many others have been promoting for many years, that lets IT professionals look after the safety, security, reliability, performance, accounting, and system configuration, while giving end users a huge amount of control over the end product.

The video introduction linked from the site explains it all better than anything on the site itself, which is mostly targeted at developers of tools and those already in the know.